1. Field of the Invention
The present invention relates to an excimer laser device, and more particularly to an excimer laser device having a controller for detecting the rotational speed of a fan for generating a laser gas flow and controlling the rotational speed of the fan at a constant level.
2. Description of the Related Art
FIG. 1 of the accompanying drawings schematically shows a conventional excimer laser device. As shown in FIG. 1, the conventional excimer laser device has a casing 101 filled with a laser gas, a preliminary ionizing electrode (not shown) disposed in the casing 101 for preliminarily ionizing the laser gas, and a pair of main discharge electrodes 102 disposed in the casing 101 for producing an electric discharge to make it possible to oscillate a laser beam. The casing 101 also houses therein a cross-flow fan 103 for producing a high-speed gas flow between the main discharge electrodes 102.
The cross-flow fan 103 has a rotatable shaft 104 projecting from opposite ends thereof and rotatably supported by a plurality of radial magnetic bearings 106, 107, 108 disposed on opposite sides of the casing 101 and an axial magnetic bearing 109 mounted on the radial magnetic bearing 106. The rotatable shaft 104 can be rotated by an induction motor 110 connected to an end thereof and disposed between the radial magnetic bearings 107, 108. The casing 101 has a pair of windows 105 on its opposite ends for emitting the laser beam out of the casing 101.
In the conventional excimer laser device, the laser gas contains a highly reactive halogen gas, e.g., a fluorine gas. Therefore, various chemical reactions are caused in the casing 101, producing impurities such as HF, CF4, etc. These impurities are responsible for a reduction in the performance of the laser beam.
In order to enable the excimer laser device to output a laser beam having an average power of several tens of watts, it is necessary to generate highly repeated electric discharges at a frequency of several kHz between the main discharge electrodes 102.
However, such often repeated electric discharges tend to cause wear on the main discharge electrodes 102 in a short period of time, resulting in a reduction in the performance of the laser beam. As a result, the excimer laser device fails to emit a laser beam which is stable over a long period of time.
One solution to the above problem is to control the discharge voltage applied between the main discharge electrodes 102, 102 and to control the pressure of the filled laser gas to maintain the laser beam performance at a constant level or higher for a long period of time.
If the pressure of the filled laser gas varies, then the load on the cross-flow fan 103 varies, and hence the rotational speed thereof also varies. Specifically, if the pressure of the filled laser gas increases, the load on the cross-flow fan 103 also increases, resulting in an increase in the slippage of the induction motor 110 which causes the rotational speed of the cross-flow fan 103 to decrease. When the rotational speed of the cross-flow fan 103 is reduced, the speed of flow of the lower gas between the main discharge electrodes 102 is reduced, with the consequence that the excimer laser device cannot oscillate at a high repetition rate.
The above drawback may be eliminated by setting the rotational speed of the cross-flow fan 103 in a high speed range for canceling out the speed reduction due to the increase of slippage of the motor 110. However, this approach is disadvantageous in that the power consumption by the cross-flow fan 103 increases when the pressure of the laser gas is high (the power consumption by the cross-flow fan 103 is proportional to the cube of the rotational speed thereof). An alternative solution is to use a slippage-free synchronous motor, which is, however, complex in structure and high in cost.
It is therefore an object of the present invention to provide an excimer laser device which is capable of keeping the rotational speed of a fan constant and maintaining uniformity in a laser gas flow between main discharge electrodes even when the pressure of a laser gas filled in a casing varies. Also, the excimer laser device can be oscillated at a high repetition rate even when the pressure of the filled laser gas varies, and consumes a reduced amount of power.
According to the present invention, there is provided an excimer laser device comprising a casing filled with a laser gas, a pair of main discharge electrodes disposed in the casing for producing an electric discharge to discharge-pump the laser gas at a high repetition rate, a fan for producing a high-speed laser gas flow between the main discharge electrodes, bearings, the fan having a rotatable shaft rotatably supported by the bearings, a motor for actuating the fan, rotational speed detecting device for detecting a rotational speed of the fan, and control device for controlling at least one of a voltage and a frequency to be supplied to the motor based on the rotational speed of the fan detected by the rotational speed detecting device, thereby to control the rotational speed of the fan at a constant level.
Because at least one of the voltage and the frequency to be supplied to the motor is controlled based on the rotational speed of the fan detected by the rotational speed detecting device, thereby to control the rotational speed of the fan at a constant level, the rotational speed of the fan can be made constant irrespective of the pressure of the sealed laser gas. The excimer laser device can therefore operate stably for oscillation at a high repetition rate. Furthermore, because the motor is operated efficiently at all times, the power consumed by the fan can be reduced.
The rotational speed detecting device may comprise a disk made of a magnetic material and having at least one slit defined therein, the disk being fixedly mounted on the rotational shaft of the fan and disposed in a hermetically sealed space communicating with the casing, a magnetic body disposed outside of the casing in confronting relationship to the disk with a can interposed therebetween, the magnetic body being narrower than the slit and having at least two protrusions, and a coil mounted on the magnetic body for inducing an electromotive force upon rotation of the disk, the control device comprising means for detecting the rotational speed of the fan from the electromotive force induced across the coil.
The rotational speed detecting device thus constructed is capable of detecting the rotational speed of the fan accurately. As the magnetic body with the coil mounted thereon is disposed outside of the casing with the can interposed therebetween, the coil which has poor corrosion resistance to the laser gas is not exposed to the laser gas, and does not deteriorate the laser gas.
The magnetic body partly or wholly comprises a permanent magnet. The magnetic body thus arranged is effective in increasing the flux density of the magnetic flux of a magnetic circuit which is made up of the magnetic body and the disk.
Accordingly, the electromotive force induced across the coil is increased, resulting in an increased ability to detect the rotational speed of the fan. Because a bias current supplied to the coil for generating the magnetic flux can be reduced or eliminated, the rotational speed detecting device can be simplified in circuit arrangement and its power consumption can be reduced.
Alternatively, the rotational speed detecting device may comprise an air flow speed sensor disposed in the casing for detecting an air flow speed, thereby detecting the rotational speed of the fan from the air flow speed detected by the air flow speed sensor.
The air flow speed sensor can directly measure an air flow speed produced by the fan, and the control device can detect the rotational speed of the fan from the detected air flow speed, and control the rotational speed of the fan in order to keep the air flow speed constant.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.